fix(files): improve handling of copy-move action

1. only show 1 loading toast instead of N for N files in batch
   operation.
2. Reuse more code to reduce duplicated logic.
3. Show the conflict picker once for all files instead of opening a new
   conflict picker for every file to copy / move.

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>

Author: susnux